1. Field of the Invention
The present invention relates to a piezoelectric transformer for use in an inverter circuit for a cold cathode tube back light of a liquid crystal panel and various power circuits for generating a high voltage. More particularly, it relates to a piezoelectric transformer which is constituted so that a piezoelectric transformer element is held in a case.
2. Description of the Related Art
A wire-wound electromagnetic transformer has been heretofore used as a transformer element for generating a high voltage in a power circuit in a device requiring a high voltage such as a deflecting device of a television or a charging device of a copier. This electromagnetic transformer is structured in such a manner that a conductor wire is wound around a magnetic core. The achievement of a high transformation ratio requires an increase in the number of turns of the conductor wire wound around the core. It is thus very difficult to miniaturize the electromagnetic transformer.
The electromagnetic transformer is being replaced by a small-sized/high-efficiency piezoelectric transformer. The piezoelectric transformer has a driving unit and a power generating unit. In this piezoelectric transformer, the application of an alternating voltage to the driving unit allows a piezoelectric transformer element to be deformed by an inverse piezoelectric effect. The deformation of the piezoelectric transformer element is then used so as to generate the voltage in the power generating unit by a piezoelectric effect.
Referring to FIG. 1, a symmetrical Rosen tertiary type piezoelectric transformer element 160 is used as the conventional piezoelectric transformer. This symmetrical Rosen tertiary type piezoelectric transformer element 160 is constituted so that electrodes are formed on both surfaces of long-plate shaped piezoelectric body 160a. One end of piezoelectric body 160a is low-impedance driving unit 161a of piezoelectric transformer element 160, while the other end thereof is low-impedance driving unit 161b of piezoelectric transformer element 160.
Input electrode 111 is formed on the front surface of driving unit 161a. The input electrode (not shown) is formed on the rear surface of driving unit 161a so that it may be opposite to input electrode 111 through driving unit 161a. Input electrode 113 is also formed on the front surface of driving unit 161b. The input electrode (not shown) is formed on the rear surface of driving unit 161b so that it may be opposite to input electrode 113 through driving unit 161b. These driving units 161a and 161b are polarized in the direction of thickness of piezoelectric body 160a.
Band-shaped output electrode 115 is partially formed between input electrode 111 and input electrode 113 on the front surface of piezoelectric body 160a so that it may pass through an intermediate portion between input electrode 111 and input electrode 113. The output electrode (not shown) is also formed on the rear surface of piezoelectric body 160a so that it may be opposite to output electrode 115 through piezoelectric body 160a.
High-impedance power generating unit 162a is provided between input electrode 111 and output electrode 115 on piezoelectric body 160a. High-impedance power generating unit 162b is also provided between input electrode 113 and output electrode 115 on piezoelectric body 160a. Power generating units 162a and 162b are polarized in the direction of paralleled arrangement of input electrode 111, output electrode 115 and input electrode 113, that is, in the longitudinal direction of piezoelectric body 160a.
In piezoelectric transformer element 160, the ends of a lead wire are soldered to node position 117 of vibration of input electrode 111, node position 118 of vibration of output electrode 115 and node position 119 of vibration of input electrode 113. The ends of the lead wire are also soldered to the node position (not shown) of the input electrode opposite to input electrode 111, the node position (not shown) of the output electrode opposite to output electrode 115 and the node position (not shown) of the input electrode opposite to input electrode 113.
The alternating voltage is input between lead wire 123 and lead wire 124, whereby the alternating voltage is applied to input electrodes 111 and 113, the input electrode opposite to input electrode 111 and the input electrode opposite to input electrode 113. Driving units 161a and 161b are thereby driven. Driving units 161a and 161b are driven, whereby the voltage is then generated in power generating units 162a and 162b by the piezoelectric effect of piezoelectric body 160a. On the other hand, lead wire 125 is soldered to output electrode 115 and the output electrode opposite to output electrode 115. The voltage generated in power generating units 162a and 162b is output from between lead wire 125 and lead wire 124. For such a symmetrical Rosen tertiary type piezoelectric transformer element, since the lead wires are connected to the node positions of vibration, a piezoelectric transformer of less loss is thus obtained.
However, when the piezoelectric transformer as shown in FIG. 1 is mass-produced, workability is disadvantageously bad during soldering the lead wire. As a solution to this problem, Japanese Patent Laid-open Publication No. 298213/96 discloses a method of holding the piezoelectric transformer element by the use of a lead terminal having spring-like characteristics. In this method, the piezoelectric transformer element is housed within a case in which the lead terminal having the spring-like characteristics is insert-molded.
Referring to FIG. 2, for the piezoelectric transformer disclosed in Japanese Patent Laid-open Publication No. 298213/96, in the same manner as the piezoelectric transformer element shown in FIG. 1, piezoelectric transformer element 260 comprising input electrode 211, input electrode 213 and output electrode 215 formed on the front surface of the piezoelectric body is used. In the same manner as the piezoelectric transformer element shown in FIG. 1, the input electrodes and output electrode are formed on the rear surface of the piezoelectric body so that they may be opposite to input electrodes 211, 213 and output electrode 215 through the piezoelectric body. Upper lead terminal 203a as a first lead terminal is insert-molded in upper case 202a, while lower lead terminal 203b as a second lead terminal is insert-molded in lower case 202b.
In such a piezoelectric transformer, piezoelectric transformer element 260 is inserted between upper case 202a and lower case 202b, so that upper case 202a is bonded to lower case 202b. In this way, piezoelectric transformer element 260 is housed in the case composed of upper case 202a and lower case 202b.
Referring to FIG. 3, in the piezoelectric transformer assembled by bonding upper case 202a to lower case 202b, input electrode 211 is electrically connected to upper lead terminal 203a on contact point 281a. Input electrode 212 opposite to input electrode 211 through the piezoelectric body is electrically connected to lower lead terminal 203b on contact point 281b. The spring-like characteristics of upper lead terminal 203a and lower lead terminal 203b cause the lead terminals to press piezoelectric transformer element 260. In such a manner, piezoelectric transformer element 260 is held in the case. Protrusion 283a is formed on the inner wall of upper case 202a, while protrusion 283b is formed on the inner wall of lower case 202b. Protrusions 283a and 283b are used for limiting the movement of piezoelectric transformer element 260.
In such a manner, the method of packaging the piezoelectric transformer by holding the piezoelectric transformer element by the spring-like characteristics of the lead terminal is very excellent in assembly and packaging of the piezoelectric transformer element.
The method of packaging the piezoelectric transformer element in the case by the use of an elastic body will be described below. The method using the elastic body is disclosed in Japanese Patent Laid-open Publication No. 36453/97 and Japanese Patent Laid-open Publication No. 342945/94.
Referring to FIG. 4, piezoelectric transformer element 391 of a secondary mode vibration using a horizontal vibration effect is used for the piezoelectric transformer disclosed in Japanese Patent Laid-open Publication No. 36453/97. This piezoelectric transformer causes one end portion of the plain-shaped piezoelectric transformer element 391 to vibrate in its width direction and the other end of piezoelectric transformer element 391 to vibrate in its width direction utilizing the vibration of one end of piezoelectric transformer element 391. The lead wire (not shown) is mounted to the electrode of piezoelectric transformer element 391 on a non-node position of vibration.
Referring to FIG. 5, Rosen secondary type piezoelectric transformer element 401 is used for the piezoelectric transformer disclosed in Japanese Patent Laid-open Publication No. 342945/94. Elastic bodies 402a and 402b are mounted by adhesive to the vibration node positions of piezoelectric transformer element 401. Lead wire 404a is mounted to the electrode on the front surface side of piezoelectric transformer element 401 on the non-node position of vibration. On the other hand, lead wire 404b is mounted to the electrode on the rear surface side of piezoelectric transformer element 401 on the non-node position of vibration. Moreover, lead wire 404c is mounted to the output-side electrode on the end of piezoelectric transformer element 401. The position on piezoelectric transformer element 401 to which lead wire 404c is mounted is also the non-node position of vibration.
This piezoelectric transformer element 401 is housed in case 403. Here, elastic bodies 402a and 402b are bonded on the inner wall of case 403 by the adhesive. In such a manner, piezoelectric transformer element 401 is held in case 403 by elastic bodies 402a and 402b.
For the piezoelectric transformer shown in FIG. 5, more specifically, lead wire 404c mounted to the output electrode on the end of piezoelectric transformer element 401 is mounted on an amplitude portion of vibration of piezoelectric transformer element 401. Therefore, more particularly, lead wire 404c considerably inhibits the vibration of piezoelectric transformer element 401.
The method of packaging the piezoelectric transformer element by the use of the elastic body, an O-ring, will be described below.
Referring to FIG. 6, Rosen secondary type piezoelectric transformer element 511 is used for the piezoelectric transformer disclosed in Japanese Patent Laid-open Publication No. 326371/94. O-ring 512a is mounted by adhesive 513 to the vibration node position of piezoelectric transformer element 511. O-ring 512b is mounted by adhesive 513 to the vibration node position differing from the vibration node position to which O-ring 512a is mounted.
On the other hand, substrate 514 is provided with hooks 515. O-rings 512a and 512b are hooked by hooks 515 corresponding to the respective O-rings. In this way, piezoelectric transformer element 511 is packaged on substrate 514 by O-rings 512a and 512b.
One end of lead wire 516a is mounted, on the non-node position of vibration, to the electrode of piezoelectric transformer element 511 on the opposite surface to substrate 514. One end of lead wire 516b is also mounted, on the non-node position of vibration, to the electrode of piezoelectric transformer element 511 on the same surface as substrate 514. One end of lead wire 516c is then mounted to the output-side electrode on the end of piezoelectric transformer element 511.
In the piezoelectric transformer shown in FIG. 6, in the same manner as the one shown in FIG. 5, lead wire 516c mounted to the output-side electrode on the end of piezoelectric transformer element 511 is mounted on the amplitude portion of vibration. Therefore, more particularly, lead wire 516c considerably inhibits the vibration of piezoelectric transformer element 511.
In a ceramic resonator in which a piezoelectric element is used as a resonator, the method of packaging the piezoelectric element within an elastic frame in which the protrusions are formed on the inner wall thereof will be described below.
Referring to FIG. 7, plate-shaped piezoelectric element 621 utilizing a mode of spread vibration is used for the ceramic resonator disclosed in Japanese Patent Laid-open Publication No. 139556/96. Positioning protrusions 623 are formed on the inner wall of elastic frame 622 for housing piezoelectric element 621. Piezoelectric element 621 is held within frame 622 by positioning protrusions 623. Electrode plates 624a and 624b incorporated in frame 622 are housed within case 625. Electrode plate 624a is arranged on the front surface of piezoelectric element 621, while electrode plate 624b is arranged on the rear surface of piezoelectric element 621. After housing piezoelectric element 621 or the like in case 625, an opening of case 625 is sealed by sealing resin 626.
In the ceramic resonator shown in FIG. 7, the protrusions for positioning piezoelectric element 621 are formed at such positions that they may inhibit the spread vibration of piezoelectric element 621.
However, since the lead wire is soldered to the node position of vibration in the above-described conventional Rosen tertiary type piezoelectric transformer, assembly workability is disadvantageously bad during the mass-production of the piezoelectric transformer.
The case for the piezoelectric transformer disclosed in Japanese Patent Laid-open Publication No. 298213/96 shown in FIGS. 2 and 3 is also provided with the lead terminals for holding the piezoelectric transformer element and the protrusions for limiting the movement of the piezoelectric transformer element. Since the piezoelectric transformer element can be packaged by housing the piezoelectric transformer element in the case, the assembly workability is good. However, the material of protrusions 283a and 283b in the case must be as hard as the material of the case in which they are molded. Therefore, when piezoelectric transformer element 260 comes into contact with protrusions 283a and 283b by an influence such as the vibration or impact, or when piezoelectric transformer element 260 is brought into contact with the case due to a positional shift, vibration characteristics of piezoelectric transformer element 260 are deteriorated and an audible sound region is vibrated. This causes the problem in which the vibration of the audible sound region is transmitted to the case and thus a noise is generated.
In the piezoelectric transformer disclosed in Japanese Patent Laid-open Publication No. 36453/97, Japanese Patent Laid-open Publication No. 342945/94 and Japanese Patent Laid-open Publication No. 326371/94, the elastic body is bonded to the node position of vibration of the piezoelectric transformer element in order to hold the piezoelectric transformer element. The piezoelectric transformer element is then mounted in the case or on the substrate by the elastic body. The lead wire is soldered to the electrode of the piezoelectric transformer element. However, the position to which the lead wire is soldered is the non-node position of vibration of the piezoelectric transformer element, and thus the lead wire cannot be mounted to the node position of vibration. Thus, disadvantageously, the mounting of the lead wire to the non-node position of vibration inhibits the vibration of the piezoelectric transformer element, resulting in the degradation of the vibration characteristics of the piezoelectric transformer element. In addition, the position to which the elastic body for holding the piezoelectric transformer element is mounted is only specified to be the node position of vibration, and the range of the mounting position is not clearly defined. The material of the elastic body is not specified in detail.
Furthermore, in the piezoelectric transformer disclosed in Japanese Patent Laid-open Publication No. 326371/94, the O-rings bonded to the piezoelectric transformer element are hooked by the hooks on the substrate, whereby the piezoelectric transformer element is packaged on the substrate. However, there is concern that vibration, impact or the like will cause the O-rings to be removed from the hooks on the substrate.
In the ceramic resonator disclosed in Japanese Patent Laid-open Publication No. 139556/96 shown in FIG. 7, positioning protrusions 623 on frame 622 are disadvantageously formed at such positions that they may inhibit the vibration of piezoelectric element 621. The shape and position of positioning protrusion 623 are not specified in detail, and the material of elastic frame 622 is not specified in detail.